1. Field of the Invention
This invention relates to an improved method of lithography. More particularly, this invention relates to a method of selective linewidth optimization of patterns on the surface of a substrate used during a lithography process.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image quality, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, and the particular reticle being used.
One technique for improving image sharpness has been proposed by Stagaman (U.S. Pat. No. 5,563,684). Stagaman observes that a conventional approach for improving image sharpness is to use a deformable chuck that flattens the wafer surface in order to conform that surface to the focal plane of the lens used. However, Stagaman further observes that the actual image pattern associated with a lens can differ from the theoretical flat focal plane of the lens, and so flattening the wafer""s surface will not necessarily improve pattern sharpness for a particular lens. Thus, Stagaman suggests an approach whereby the actual focal pattern of a lens is determined. A flexible chuck is then used to conform the surface of the wafer to the actual focal pattern of the lens, thereby improving average image sharpness. While this approach may prove satisfactory in some processing environments, the instant inventors have discovered that correcting for average image sharpness is not the best approach in many processing environments. Rather, a method for correcting for particular types of distortions is needed, since required device tolerances are related to specific factors and not necessarily to average image sharpness.
Particular types of distortion within a lithographic system may be characterized by linewidth control parameters. Linewidth control parameters of any given line or feature within a printed pattern vary as a result of optical capabilities of the lithography apparatus used, particular characteristics of the reticle, focus setting, light dose fluctuations, and the like. The instant invention uses focus offset coefficients to change the focus at points within a slot, like that discussed above, to compensate for the linewidth control parameter variations introduced by the factors contributing to such variations. A set, or sets, of focus offset coefficients is generated for a particular lithography apparatus, depending on the number of linewidth control parameters for which correction is desired.
The disclosed invention can be used for optimization of features other than lines, since linewidth control parameters can be used to characterize features other than lines (e.g., contacts and the like). Furthermore, while the instant invention is desribed in terms of optimizing linewidth control parameter through the use of focus offset coefficients, any type of systemic problem that can be compensated through the use of focus offsets is within the scope of this invention. For example, retical bi-refringence, dose control and illumination properties, and vibration are all systemic problems capable of compensation through the use of focus offset coefficients, and thus their compensation through the use of the method and system of the instant invention is within the scope of this disclosure.
In a preferred embodiment, the instant invention is a method of selective linewidth optimization within a lithographic system including the steps of selecting a linewidth control parameter for optimization and setting multiple focuses within a slot being scanned in the lithographic system so as to optimize the selected linewidth control parameter. The linewidth control parameter selected can include horizontal-vertical bias (xe2x80x9cH-V biasxe2x80x9d), group-to-isolated bias (xe2x80x9cG-I biasxe2x80x9d), and critical-dimension through-focus (xe2x80x9cCD through-focusxe2x80x9d). More than one such linewidth control parameter can be selected for optimization.
In order to set multiple focuses across a substrate, a flexible chuck is flexed, altering the focus at multiple points within the slot, thus optimizing the selected linewidth control parameter or paramters. The particular focus-settings are generating by calibrating a lithographic apparatus, a reticle, or a lithographic apparatus with reticle. Such calibrating can include the printing of a field through-focus or the use of an aerial image monitor to determine linewidths through-focus for a particular feature type within a slot. Separate calibrations can be done for different feature types (e.g., contacts and signal lines), or a single calibration can be performed.
In addition to setting multiple focuses at points within the slot, dynamically adjusting the focus at a particular slot location along the scan is also disclosed. A focus offset coefficient matrix is generated that can include multiple focus offset coefficients for points within a slot and at multiple slot locations along the scanning direction.
Also disclosed is a method of generating sets of focus offset coefficients for use in a lithographic system. The disclosed method includes a step of calibrating a lithography apparatus, a reticle, or a lithography apparatus with reticle. Such calibration produces data from which the sets of focus offset coefficients can be generated. These focus offset coefficients can include those corresponding to points within a slot and can additionally include focus offset coefficients for multiple slot points along a scanning direction. During calibration, linewidths are measured optically, with electrical probes, or with an aerial image monitor. Model data from sufficiently accurate models can also be used to generate focus offset coefficients.
The focus offset coefficients can be determined by plotting curves from the data gathered during calibration or by using a computation device or element to determine the sets of focus offset coefficients directly from the data.
Also disclosed is a system for selective linewidth optimization during a lithographic process. Such a system can include a projection system, a flexible chuck, and a focus offset controller connected to the projection system and the flexible chuck. The focus offset controller is configured to set multiple focuses within a slot being scanned during the lithographic process so as to optimize a linewidth control parameter. Such a system can also include a wafer stage upon which the flexible chuck is disposed and can be configured to set multiple focuses at a single slot point for different slot locations along a scanning direction.